Frequency control devices are known to include various types of crystal oscillators. A typical quartz crystal oscillator utilizes several components including a piezoelectric element, an integrated circuit, capacitors, inductors, resistors, etc. These frequency control devices are commonly found in electronic communication devices such as, cellular phones, pagers, radios and wireless data devices. As consumer demand continually drives down the size and cost of this equipment, the need for oscillators to be smaller and less costly has become even greater.
The most sensitive component in a crystal oscillator is the piezoelectric element. Typically, this element is independently sealed in a hermetic package. The purpose of encapsulation is to passivate the integrated circuit (IC) die of the oscillator from the effects of the environment. This die is fragile and must be protected from excessive thermal and mechanical stresses and strains. Also, the die must be protected from exposure to chemicals including moisture, oxygen, acids, corrosives, etc. Previously, this passivation process had been accomplished by thermoset encapsulation techniques, ceramic packaging, or potting epoxies on printed circuit boards.
The most common method for passivation of an IC is thermoset plastic encapsulation by transfer molding. In this technique an IC is mounted and wire bonded to a conductive lead frame. Then the lead frame is suspended in a two piece mold cavity and a thermoset resin is slowly injected under high temperature and pressure to form a thermoset plastic which encapsulates the part. This is commonly referred to as transfer molding. This process is low cost, but it does have problems. Due to the temperatures and pressures used in this molding process, piezoelectric devices are thermally and mechanically stressed causing undesirable frequency shifts. Also, this method may cause occasional damage to the IC or its wirebonds. In addition, some thermoset resins can outgas chlorine and ammonia by-products which can corrode the metalization on the IC. Also, thermosets tend to be hydrophilic and may lose their effectiveness against moisture infiltration.
Alternatively, potting epoxies have been used in some applications to directly encapsulate IC dies on various substrates. Potting techniques sometimes incorporate complicated structures and have microcracking problems due to the differences in thermal expansion coefficients of the materials used. However, this technique is of a lower cost than ceramic packaging. Some prior art potting techniques have utilized a dam and an underlayment to contain a potting compound over a lead frame. However, there are some problems with this approach. First, an additional B-stage epoxy process is needed to provide a moisture barrier between the leads and the dam. Second, the use of a deposited, preformed or machined dam increases the cost of the product. Third, more packaging components are used. The present invention can solve many of these problems by the use of a pre-molded thermoplastic shell to contain an encapsulant.
In ceramic packaging techniques, the IC die is first mounted to a ceramic substrate and this ceramic substrate is placed within a hermetic package. This hermetic package may take the form of a welded metal can with glass-filled feedthroughs for the external electrical connections, or it may take the form of a single or multi-layered ceramic substrate that is sealed to a single or multi-layered ceramic lid by means of an epoxy or glass frit. Alternatively, the ceramic substrate may be sealed with a brazed or welded steel alloy lid. Although the above technique has been successful, these packages are structurally complex and relatively expensive.
A significant portion of the cost of a quartz crystal oscillator is in its packaging. These oscillators typically have higher material and labor costs than a similarly packaged IC. Therefore, oscillator scrap costs due to yield losses are to be avoided if at all possible. Previously, the packaging used for crystal oscillator devices was typically ceramic packaging. This packaging has demonstrated good yields and therefore low incurred scrap costs. However, the ceramic structures themselves, though relatively effective, are complicated and have a higher inherent cost. Cost reduction can be achieved if the packaging for these oscillators can be simplified without sacrificing yield.
There is a need for an improved encapsulation process for making crystal oscillators that is: (i) low cost and high yield; (ii) minimizes the number of process steps and separate packaging components; (iii) does not require external machining of sealing components; (iv) is robust under environmental testing; (v) minimizes problems from differing thermal expansion coefficients of package materials; (vi) reduces the migration of moisture or oxygen into the package; and (vii) is readily manufacturable without custom equipment or added costs.